1. Field of the Invention
This invention relates to ventilation systems for use in supplying breathable gas to a patient and more particularly to a ventilation system that supplies treated gas under variable pressure and in accordance with selected modes.
2. State of the Art
The use of mechanical ventilators to deliver a mixture of oxygen and air to a patient's airway in medical treatment situations including, for example, intensive care facilities is known. Known ventilation devices are configured to deliver large volumes of therapeutic gases under positive pressure at frequencies that generally match the normal inhalation rate of the patient. For example, a volume ventilator may deliver a measured quantity of gas at a fixed rate or in response to patient respiratory effort. Use of large gas volume systems has typically been limited to pediatric and adult patients.
Another type of ventilator presently in use typically for neonatal and pediatric patients is the time cycled, pressure limited device schematically depicted in FIG. 1. As shown, air and oxygen are blended to a desired ratio by a BLENDER. After passing through a regulator, the mixed gas is heated and humidified in a HUMIDIFIER. The mixed gas then flows from the HUMIDIFIER through the inhalation limb of a patient breathing circuit to a WYE connector. The WYE connector has one end connected to an endotracheal tube for positioning in the trachea of a patient. In turn, the mixed gas is presented to the patient for inhalation through the endotracheal tube. Upon exhalation and during periods where inhalation has been terminated, the mixed gas and exhaust gas then return through the exhalation limb of the breathing circuit to a variable orifice EXHALATION VALVE.
When the EXHALATION VALVE is closed, the flow of the mixed gas into the exhalation limb is blocked or diverted from the exhalation circuit into the endotracheal tube. When the EXHALATION VALVE is open, the gas in the exhalation limb escapes to atmosphere and the patient is free to exhale.
As depicted, the EXHALATION VALVE may be a pneumatically controlled diaphragm valve. Should the pressure in the airway exceed the closing gas pressure of the EXHALATION VALVE, the EXHALATION VALVE would automatically open to relieve the over pressure. Mechanical ventilation may be initiated by varying the control pressure to the EXHALATION VALVE so that the pressure in the airway to the patient varies between a low setting termed the positive end-expiratory pressure (PEEP) to a higher value termed the peak inspiratory pressure (PIP). For the time-cycled pressure-limited ventilator described, the EXHALATION VALVE control pressure is varied from high to low at some fixed rate and the airway pressure automatically limited as described. The bias gas flow to the EXHALE CONTROL VALVE may be adjusted to regulate the EXHALATION VALVE and in turn increase or decrease the PIP pressure, the volume delivered and the flow rate of gas delivered with each inspiratory cycle. Typically the bias gas flow pressure approximates the desired PIP. As a result, the large volume of relatively low pressure gas present in the breathing circuit dictates a relatively slow response time to changes in the control signals to the EXHALATION VALVE from the EXHALE VALVE CONTROL. As a result, systems such as the one illustrated in FIG. 1 typically can not respond to very rapid changes in demand for flow or pressure as in the beginning of inhalation or exhalation. The rapid changes can arise so fast (e.g., a few milliseconds) that the changes are sometimes stated to be "instantaneous." The time lag in supplying sufficient gas or pressure increases the work of spontaneous breathing for the patient when some respiratory therapies are provided which are intended to help limit the work required of the patient to breath but do not in fact assist in limiting the work to the degree desired.
In the treatment of certain respiratory illnesses, it is known to be advantageous to deliver pulses of air and oxygen at frequencies much greater than the normal respiratory rate of the patient. So called high frequency ventilators are designed to deliver small volumes of gas at relatively high rates which typically are greater than one hundred and fifty (150) breaths per minute. Examples of such systems are described in U.S. Pat. Nos. 4,481,944, 4,538,604, and 5,239,994. One known high frequency ventilator uses a jet nozzle to direct gas into the airway of the patient.
Known high frequency ventilator systems do not generally incorporate humidifiers or other components employed in mechanical ventilation. Further, present ventilation systems are bulky and difficult to transport other than on some sort of wheeled cart. Further, such systems do not offer rapid response times along with the ability to select various modes of operation at different pressures and rates.